1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile machine which performs color image formation using an electrophotographic system and an image forming method.
2. Related Background Art
An image forming apparatus which forms a color image is put to practical use, for example, by sequentially superposing and transferring color toner images formed on a photosensitive drum which is an image bearing member, to a recording material such as paper held on a transfer drum (transfer film), as the image forming apparatus which forms the color image.
In such an image forming apparatus, an electrostatic latent image formed on a photosensitive drum based on an input image signal is developed with first color toner (for example, cyan) to form a toner image and this toner image is transferred to a recording material such as paper held on a transfer drum (transfer film). A color image can be obtained by also applying this transfer process to other three colors, that is, the toner of each color of magenta, yellow and black in the same manner, and by sequentially superposing and transferring 4-color toner images on a recording material.
Incidentally, if the color image forming apparatus described above is a color copying machine, for example, fine density unevenness is easily produced on a color image formed through copying in an original with low density and halftone such as a photograph original although it is hardly produced in a character original. In general, this density unevenness is called "coarseness".
In the density unevenness described above, cyan and magenta are visually easy to stand out clearly in particular among colors of cyan, magenta and yellow.
This unevenness cannot be found in ink-jet and printing. The biggest problems are an unstable element of image quality which cannot be forecast and a low frequency noise occurring on a macro basis when a large number of fine toner particles having a particle diameter of 5 to 10 .mu.m forth a dot contour by being distributed at random. This is described below in detail.
FIG. 14 and FIG. 15 are output images of a practical electrophotographic photograph system and an ink-jet system respectively. FIG. 14 shows an appearance of fine toner (on copying paper) which forms a dither dot in the electrophotographic system by an enlarged diagram. FIG. 15 is an enlarged diagram of almost the same scale in which a similar dot was output on dedicated glossy paper by an ink-jet system (Epson PM700). Accordingly, for the electrophotographic system, it comes out that a dot does not have such a smooth contour shape as in the ink-jet system and a large number of fine toner particles having a fine particle diameter of 5 to 10 .mu.m form a dot contour by being distributed at random. Further, the finish of a dot is also not the same. There are dots whose density is low or high and whose dot diameter is small or large. For a shape, it is not far from round (cylindrical) but is oval (distorted), and there is no shape which is equal one after another. The variation of these factors is almost random and contains considerably low frequency components. As a result, the variation becomes a cause of noise which can be seen with eyes.
The difference between toner density and paper density discriminates this noise. In particular, in comparison with an ink-jet system, this system remarkably receives the effect of an optical dot gain due to the distribution of an infinite amount of fine toner.
The main cause of the phenomenon described above is that an electrophotographic system uses a fine toner particle to form a dot. Further, there are various promoting causes. That is, they are un-sharpening of dot data in the latent image to developing to transfer process of the electrophotographic process, scattering of irregular ton.RTM.r resulting from values of physical properties (electrical resistance and surface roughness) of copying paper and a phenomenon resulting from the adhesion in the developing process described below.
For a one-component developer, the adhesion force between toner and a developing sleeve, and for a two-component developer, the adhesion force between the toner and a carrier (mainly reflection force of the toner on a developer bearing member) are strong, whereas the charging distribution of the toner is uneven. Therefore, if these toner are peeled off at a developing bias voltage and made to fly onto a photosensitive drum, such unstable image formation as the toner at one place is easy to fly and the toner at another place is hard to fly occurs, and unevenness will occur in the formation of a dot.
On the other hand, because in the gradation ink process of such an ink-jet system as can be seen in Japanese Patent Application Laid-Open No. 58-39468, the ink-jet system itself is simple and the performance of dedicated paper which supports the current high quality image is excellent, the problem of such an electrophotographic system as described above does not occur.
Therefore, in the improvement of graininess which is the effect of gradation ink used in an ink-jet system, it is proved that light toner has a more greater effect of an electrophotographic system on the remarkable low frequency noise resulting from the "fluctuation of toner density at which a dot is formed", "fluctuation of a dot area" and "fluctuation of a dot shape" described above than the ink-jet system.
Moreover, it is proved that the introduction of light toner to an electrophotographic system brings about an innovative progress in that an optical dot gain which did not cause any problem in an ink-jet system has caused a great hindrance when high quality is aimed at in the electrophotographic system which uses an indefinite amount of fine toner, too.
As shown in FIG. 12, in a practical copy line "PCL" against ideal copy, the density of a copy (reflection density of a copy) shows a characteristic that the density is lower than the density of an original (reflection density of an original) in a low density area and the density of the copy suddenly increases from the vicinity of intermediate density of the density of the original and then the maximum density is reached quickly. This results from the adhesion force in the developing process described previously.
The maximum density (maximum reflection density) is additionally described below. FIG. 12 shows a phenomenon which can he described in accordance with FIG. 13. That is, it is assumed that the lower vertical axis shows the density data of an original or digital image data read by a scanner or the like and the horizontal axis shows the voltage difference (developing contrast) between the potential of a latent image formed by charging to laser exposure after the information has been converted to data for optical writing and the voltage applied to a developing bias voltage. The correction is made to the original density date to developing contrast according to a case, but if a straight line without correction is used, the curve of FIG. 12 is controlled by a developing system, that is, an amount of toner coat on a developing sleeve, an amount of toner charging, the capacitance of a photosensitive body, the peripheral ratio between a process speed and the developing sleeve, a developing bias voltage and the like.
As shown in FIG. 13, if the direct current component of the developing bias voltage is changed arid the developing contrast increases, a curve is weakened because electrical capacitance or an amount of toner which can be supplied is saturated. That is, in the image forming apparatus, the maximum density on transfer paper the toner can produce is reached.